PROJECT SUMMARY Zika virus (ZIKV) is a flavivirus, primarily transmitted to humans by the bite of infected mosquitoes and can infect a variety of cells in the placenta and fetal brain. An unexpected surge in congenital microcephaly cases in Brazil suggested that the virus was teratogenic in pregnancy, which we and others confirmed in nonhuman primate models. Although the initial epidemic was geographically limited, outbreaks will recur, and the US remains at risk for a ZIKV epidemic. Thus, there is an urgent and enduring need to investigate therapeutics and vaccines to prevent pregnancy infections that can permanently injure the fetus. We have developed a highly relevant nonhuman primate (NHP, Macaca nemestrina, pigtail macaque) model of the congenital ZIKV syndrome in which we have identified a consistent injury pattern to the fetal brain. We can now leverage this model to enable investigation of novel therapeutics for protection of the fetal brain from ZIKV infection during pregnancy. Our central hypothesis is that prophylactic Sofosbuvir will prevent maternal-fetal transmission of ZIKV in a pregnant nonhuman primate model. Sofosbuvir (Sovaldi, Gilead Sciences, Inc.) is an antiviral therapeutic licensed for treatment of chronic Hepatitis C infection, which targets ZIKV NS5, a non-structural protein important for replication of the viral genome. We have chosen to test Sofosbuvir due to evidence for: 1) inhibition of ZIKV infection in a cell lines and pregnant mice, 2) a solid safety profile with no known risks in human pregnancy, 3) a tiered pricing structure for low- and middle-income countries, and 4) possible cross-viral protection due to homologies in viral NS5 proteins. In one Aim, we will determine the ability of prophylactic Sofosbuvir to prevent maternal ZIKV viremia and congenital infection in a pregnant NHP model. We will administer oral Sofosbuvir daily for 3 days prior to and 7 days after ZIKV challenge at ~100 days gestation (second trimester) and perform Cesarean section 7 days after inoculation, followed by necropsy of the dam and fetus. The primary study outcome is detection of ZIKV RNA in the dam sera on Days 2-3 post-inoculation, which is a high-risk state for maternal-fetal transmission. Secondary outcomes include: 1) detection of replicating virus or viral nucleic acids in maternal, placental or fetal tissues or fluids (amniotic fluid, cord blood) and 2) fetal neuropathology focused on identification of periventricular injury. If successful, prophylactic Sofosbuvir could represent the first therapeutic option to protect pregnancies from ZIKV infection for women living in areas with local mosquito-borne transmission or for pregnant women (or their sexual partners) traveling to these areas. Further, a strategy to prevent ZIKV viremia in the adult mother could be translated to other vulnerable adult populations (e.g. elderly, immunocompromised adults) with increased susceptibility to neurologic complications associated with ZIKV infection. These studies will allow us to obtain pilot data in preparation for an R01 proposal to determine the efficacy, pharmacokinetics (in pregnancy) and fetal safety of using Sofosbuvir to prevent congenital ZIKV infections.